Re: RFC: Remove empty output sections

["H. J. Lu" <hjl@lucon.org>]

On Tue, Mar 15, 2005 at 08:45:44AM +1030, Alan Modra wrote:
> On Mon, Mar 14, 2005 at 08:10:48AM -0800, H. J. Lu wrote:
> > On Mon, Mar 14, 2005 at 07:05:15PM +1030, Alan Modra wrote:
> > > On Sun, Mar 13, 2005 at 10:31:41PM -0800, H. J. Lu wrote:
> > > > @@ -4883,7 +4893,7 @@ lang_process (void)
> > > >      lang_check_section_addresses ();
> > > >  
> > > >    /* Final stuffs.  */
> > > > -
> > > > +  lang_mark_used_section ();
> > > >    ldemul_finish ();
> > > >    lang_finish ();
> > > >  }
> > > 
> > > Isn't this too late to strip sections?  What happens if one of the
> > > sections stripped has a dynamic section symbol?
> > 
> > An empty section has a dynamic section symbol. Do you have a testcase
> > for that?
> 
> Easy.  This also demonstrates another potential problem with removing
> empty sections;  Their alignment can affect layout of other sections.
> 

Here is the output from readelf -Sls after removing empty sections.
Removing an empty section will certainly change the layout of other
sections. I don't think it should be a problem. I believe that
anything depending on alignment of an empty section is broken.

3 local entries in .dynsym have UND index since they are removed. It
is hard to remove them since it is done too late. I don't think it
should cause any probleme though.


H.J.
-----
There are 11 section headers, starting at offset 0x2164:

Section Headers:
  [Nr] Name              Type            Addr     Off    Size   ES Flg Lk Inf Al
  [ 0]                   NULL            00000000 000000 000000 00      0   0  0
  [ 1] .text             PROGBITS        00000000 001000 000001 00  AX  0   0  4
  [ 2] .dynsym           DYNSYM          00001000 002000 000070 10   A  3   4  4
  [ 3] .dynstr           STRTAB          00001070 002070 00000a 00   A  0   0  1
  [ 4] .hash             HASH            0000107c 00207c 000030 04   A  2   0  4
  [ 5] .data             PROGBITS        000010ac 0020ac 000004 00  WA  0   0  4
  [ 6] .dynamic          DYNAMIC         000010b0 0020b0 000058 08  WA  3   0  4
  [ 7] .got.plt          PROGBITS        00001108 002108 00000c 04  WA  0   0  4
  [ 8] .shstrtab         STRTAB          00000000 002114 00004f 00      0   0  1
  [ 9] .symtab           SYMTAB          00000000 00231c 0000d0 10     10  12  4
  [10] .strtab           STRTAB          00000000 0023ec 000020 00      0   0  1
Key to Flags:
  W (write), A (alloc), X (execute), M (merge), S (strings)
  I (info), L (link order), G (group), x (unknown)
  O (extra OS processing required) o (OS specific), p (processor specific)

Elf file type is DYN (Shared object file)
Entry point 0x0
There are 2 program headers, starting at offset 52

Program Headers:
  Type           Offset   VirtAddr   PhysAddr   FileSiz MemSiz  Flg Align
  LOAD           0x001000 0x00000000 0x00000000 0x01114 0x01114 RWE 0x1000
  DYNAMIC        0x0020b0 0x000010b0 0x000010b0 0x00058 0x00058 RW  0x4

 Section to Segment mapping:
  Segment Sections...
   00     .text .dynsym .dynstr .hash .data .dynamic .got.plt 
   01     .dynamic 

Symbol table '.dynsym' contains 7 entries:
   Num:    Value  Size Type    Bind   Vis      Ndx Name
     0: 00000000     0 NOTYPE  LOCAL  DEFAULT  UND 
     1: 00000000     0 SECTION LOCAL  DEFAULT    1 
     2: 00000000     0 NOTYPE  LOCAL  DEFAULT  UND 
     3: 000010ac     0 SECTION LOCAL  DEFAULT    5 
     4: 00000000     0 NOTYPE  LOCAL  DEFAULT  UND 
     5: 00000000     0 NOTYPE  LOCAL  DEFAULT  UND 
     6: 000010b0     0 OBJECT  GLOBAL DEFAULT  ABS _DYNAMIC

Symbol table '.symtab' contains 13 entries:
   Num:    Value  Size Type    Bind   Vis      Ndx Name
     0: 00000000     0 NOTYPE  LOCAL  DEFAULT  UND 
     1: 00000000     0 SECTION LOCAL  DEFAULT    1 
     2: 00001000     0 SECTION LOCAL  DEFAULT    2 
     3: 00001070     0 SECTION LOCAL  DEFAULT    3 
     4: 0000107c     0 SECTION LOCAL  DEFAULT    4 
     5: 000010ac     0 SECTION LOCAL  DEFAULT    5 
     6: 000010b0     0 SECTION LOCAL  DEFAULT    6 
     7: 00001108     0 SECTION LOCAL  DEFAULT    7 
     8: 00000000     0 SECTION LOCAL  DEFAULT    8 
     9: 00000000     0 SECTION LOCAL  DEFAULT    9 
    10: 00000000     0 SECTION LOCAL  DEFAULT   10 
    11: 00001108     0 OBJECT  LOCAL  HIDDEN  ABS _GLOBAL_OFFSET_TABLE_
    12: 000010b0     0 OBJECT  GLOBAL DEFAULT  ABS _DYNAMIC